1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to support structures for electrical transformers.
2. Description of the Prior Art
In conventional constructions of electrical inductive apparatus, such as electrical transformers, the magnetic core is generally comprised of top and bottom yokes which connect one or more leg sections on which an electrical winding assembly or coil is axially disposed. The magnetic core is formed of stacked laminations of magnetic material held in assemblied relation by top and bottom end frame members which are disposed along the top and bottom yokes, respectively.
In certain transformer constructions, such as those having core and coil assemblies with a rectangular cross-sectional configuration, additional side braces or end plates are connected between the top and bottom end frame members and are disposed in abutting relation with a portion of the outermost turn of the electrical winding assembly to prevent movement or rounding out of the rectangular winding under stresses incident to short circuit or transient conditions. As shown in U.S. Pat. No. 3,792,395, issued to G. Michel, and assigned to the assignee of this application, such end plates consist of thick steel slabs or plates which are welded to the top and bottom end frames adjacent opposing ends of the coil assembly to form a rigid support structure.
Although satisfactory in operation, such end plate construction poses problems in view of the recent trend to higher voltages and correspondingly higher ratings for electrical apparatus, such as transformers; which result in increased short circuit and transient forces acting upon the transformer. In view of these increased forces, the beam strength of the end plates must be increased in order to prevent the movement or rounding out of the windings of the transformer. The normal approach used to increase the strength of the end plates is to increase their thickness. However, this increases the overall weight of the transformer and results in higher shipping costs and handling difficulties. In addition, prior art type end plates require considerable machining to round off the sharp corners of the rectangular plate and thereby prevent corona inception at these points.
The trend toward higher voltages in electrical distribution systems also poses problems for the mechanical integrity of the magnetic cores of electrical inductive apparatus, such as transformers. In a typical construction, the yokes and legs of a magnetic core are joined together by a so called step-lap joint, such as that described in greater detail in U.S. Pat. No. 3,153,215 which is assigned to the assignee of the present application. In such a configuration, the joint between the mitered or diagonally cut ends of the legs and yokes in each layer of the core laminations are increasingly offset from the joints in adjacent layers in a stepped or progressive pattern. The increased forces acting upon this type of magnetic core, due to higher than normal operating voltages, result in movement or separation of the core legs relative to the yokes at the joint region. Prior art type support structures have proved to be insufficient in maintaining the integrity of the magnetic core under the higher operating voltages required and, thus, limit the maximum rating for rectangular core and coil type transformers.
In view of the foregoing discussion, it is desirable to provide an electrical inductive apparatus, such as a transformer having rectangular core and coils, that has a higher rating than previously attainable with this type of transformer construction.
In order to provide a transformer with a higher rating than previously attainable using prior art methods, it is of primary importance to eliminate or substantially restrict the movement of the winding and magnetic core incident to forces caused by short circuit and transient conditions. It would, thus, be desirable to provide a transformer having an improved support structure that provides increased strength to resist movement of the winding without a corresponding increase in the weight of such support structure. It would also be advantageous to provide a transformer having an improved support structure that resists or substantially eliminates movement of the legs of the magnetic core relative to the core yokes.